Andromeda chained to the Box – Dynamical Models for M31: Bulge & Bar

• DOI: 10.1093/mnras/stw3294
• Publication Date: 2016-12-19T20:05:54Z
• Authors: Matias Blaña Díaz; Christopher Wegg; Ortwin Gerhard; Peter Erwin; Matthieu Portail; Michael Opitsch; Roberto Saglia; Ralf Bender
• Keywords: Astrophysics of Galaxies (astro-ph.GA); 0103 physical sciences; FOS: Physical sciences; Astrophysics - Astrophysics of Galaxies; 01 natural sciences

Andromeda is our nearest neighbouring disk galaxy and a prime target for detailed modelling of the evolutionary processes that shape galaxies. We analyse the nature of M31's triaxial bulge with an extensive set of N-body models, which include Box/Peanut (B/P) bulges as well as initial classical bulges (ICBs). Comparing with IRAC 3.6$��m$ data, only one model matches simultaneously all the morphological properties of M31's bulge, and requires an ICB and a B/P bulge with 1/3 and 2/3 of the total bulge mass respectively. We find that our pure B/P bulge models do not show concentrations high enough to match the S��rsic index ($n$) and the effective radius of M31's bulge. Instead, the best model requires an ICB component with mass $M^{\rm ICB}=1.1\times10^{10}{\rm M_{\odot}}$ and three-dimensional half-mass radius $r_{\rm half}^{\rm ICB}$=0.53 kpc (140 arcsec). The B/P bulge component has a mass of $M^{\rm B/P}=2.2\times10^{10}{\rm M_{\odot}}$ and a half-mass radius of $r_{\rm half}^{\rm B/P}$=1.3 kpc (340 arcsec). The model's B/P bulge extends to $r^{\rm B/P}$=3.2 kpc (840 arcsec) in the plane of the disk, as does M31's bulge. In this composite bulge model, the ICB component explains the velocity dispersion drop observed in the centre within $R<br/>20 pages, 13 figures, 1 table. Accepted for publication in MNRAS<br/>